Abstrict A mass air flow meter for an internal combustion engine is described
in which the mass air flow into the engine is determined by the
measured throttle angular position, the sensed pressure above the
throttle and a stored schedule of values dependent upon the ratio
of the pressure below the throttle to the pressure above the throttle.
Claims The embodiments of the invention in which an exclusive property
or privilege is claimed are defined as follows:
1. An air meter for an internal combustion engine having an intake
manifold, an induction passage opening from an air inlet to the
intake manifold and including a throttle bore and a variable position
throttle in the throttle bore for varying the effective area of
the induction passage to regulate the air flow into the intake manifold,
the air meter comprising, in combination:
means effective to measure the value P.sub.1 of the pressure in
the induction passage upstream of the throttle;
means effective to measure the value P.sub.2 of the pressure in
the induction passage downstream of the throttle;
means effective to measure throttle position;
circuit means responsive to the ratio P.sub.2 /P.sub.1 effective
to provide a schedule of values D that substantially satisfies the
expression C.sub.SS .sqroot.1-P.sub.2 /P.sub.1 for subsonic air
flow through the induction passage opening defined by the throttle
and substantially equal to C.sub.S for sonic air flow through the
induction passage opening defined by the throttle, where C.sub.SS
is a pressure ratio dependent discharge coefficient and C.sub.S
is the sonic flow discharge coefficient;
circuit means responsive to the measured throttle position effective
to provide a schedule of values A representing the effective area
of the induction passage defined by the throttle;
means responsive to the scheduled values of D and A and the measured
value P.sub.1 effective to provide an air flow signal that substantially
satisfies the expression A.multidot.K.sub.1 .multidot.P.sub.1 .multidot.D
where K.sub.1 is a constant, the air flow signal being a substantial
measure of the mass rate of air flow inducted into the intake manifold
through the induction passage.
2. An air meter for an internal combustion engine having an intake
manifold, an induction passage opening from an air inlet to the
intake manifold and including a throttle bore and a variable position
throttle in the throttle bore for varying the effective area of
the induction passage to regulate the air flow into the intake manifold,
the air meter comprising, in combination:
means effective to measure the value P.sub.1 of the pressure in
the induction passage upstream of the throttle;
means effective to measure the value P.sub.2 of the pressure in
the induction passage downstream of the throttle;
means effective to measure throttle position;
first storage means having memory locations addressable by the
pressure ratio P.sub.2 /P.sub.1 and having an air flow driving function
value D stored at each memory location representing the pressure
ratio influence on the air flow through the induction passage, the
stored value of D at each memory location substantially satisfying
the expression C.sub.SS .sqroot.1-P.sub.2 /P.sub.1 for subsonic
air flow through the induction passage opening defined by the throttle
and substantially equal to C.sub.S for sonic air flow through the
induction passage opening defined by the throttle, where C.sub.SS
is a pressure ratio dependent discharge coefficient and C.sub.S
is the sonic flow discharge coefficient;
second storage means having memory locations addressable by the
value of throttle position and having a value A stored at each memory
location representing the effective area of the induction passage
opening defined by the throttle;
means effective to recall the values D and A from the first and
second memories in accord with the measured values P.sub.1 P.sub.2
and throttle position; and
means effective to provide an air flow signal that substantially
satisfies the expression A.multidot.K.sub.1 .multidot.P.sub.1 .multidot.D,
where K.sub.1 is a constant, the air flow signal being a measure
of the air inducted into the intake manifold through the induction
passage.
Description This invention relates to a mass air flow meter and specifically
relates to such a meter for an internal combustion engine having
an intake manifold, an induction passage opening from an air inlet
to the intake manifold and including a throttle bore and a variable
position throttle in the throttle bore for varying the effective
area of the induction passage to regulate the air flow into the
intake manifold.
Numerous systems have been proposed for measuring the mass rate
of air flow into an internal combustion engine. Generally, these
systems require an air flow sensing element positioned in the air
stream to sense air flow. For example, some prior systems provide
for a vane that is moved by the air flow to an angular position
which is a measure of air flow. Other systems employ a constant
temperature anemometer positioned in the air stream.
As opposed to the foregoing form of air flow sensors, in the present
invention, the mass rate of air flow into an internal combustion
engine is determined without the addition of an air flow sensing
element and without differentiating between sonic and subsonic air
flows by using the position of the throttle valve in the engine
air induction passage, the air pressure above and below the throttle
valve and a stored schedule of values of a pressure ratio dependent
air flow driving function. Accordingly, the general object of this
invention is to provide an improved mass rate of air flow meter
for an internal combustion engine that does not require the addition
of an air flow sensing element in the air induction passage of the
internal combustion engine.
It is another object of this invention to provide an improved mass
air flow sensor for an internal combustion engine employing the
throttle valve of the internal combustion engine in conjunction
with a stored schedule of pressure ratio dependent air flow driving
functions.
It is another object of this invention to provide for a mass air
flow meter for an internal combustion engine having an air throttle
valve utilizing the angular position of the throttle valve, the
pressures on each side of the throttle valve, and a stored schedule
of air flow driving functions and which does not require the distinction
between sonic and subsonic air flows through the throttle valve.
These and other objects of this invention may be best understood
by reference to the following description of a preferred embodiment
and the drawings in which:
FIG. 1 illustrates a digital system used with an internal combustion
engine for providing mass air flow measurements in accord with the
principles of this invention;
FIGS. 2 and 3 are diagrams illustrative of the operation of the
digital system of FIG. 1; and
FIG. 4 is a diagram of the pressure ratio dependent driving function
stored in the ROM of FIG. 1 and which is used in determining mass
air flow in accord with the principles of this invention.
Referring to FIG. 1 an internal combustion engine 10 includes
an induction passage opening from the atmosphere into an intake
manifold 12. The intake passage includes a throttle bore 14 having
a variable position throttle 16 therein for varying the effective
area of the induction passage to regulate the air flow into the
intake manifold 12.
The mass air flow measurement system of this invention is described
in conjunction with a fuel control system for the engine 10 wherein
a fuel injector 18 is controlled to inject fuel into the throttle
bore 14 above the throttle 16. The amount of fuel injected is based
on the measured mass air flow into the engine so as to establish
a desired air/fuel ratio.
The digital system for measuring the mass air flow into the engine
10 and for controlling the fuel injector 18 includes a microprocessor
20 which executes an operating program permanently stored in an
external read-only memory (ROM) 22. The ROM 22 also contains lookup
tables addressed in accord with selected engine parameters as will
be described in determining the mass air flow into the engine 10.
Internal to the microprocessor 20 are conventional counters, registers,
accumulators, flag flip-flops, etc. Such a microprocessor may take
the form of a Motorola MC-6800 Series Microprocessor. The digital
system also includes a random access memory (RAM) 24 into which
data may be temporarily stored and from which data may be read at
various address locations determined in accord with the program
stored in the ROM 22. A clock oscillator 26 which establishes the
timing of the digital system, supplies a clock signal to the microprocessor
20 and to a divider 28 which issues a periodic interrupt pulse to
a maskable interrupt A input of the microprocessor 20. These interrupt
pulses may be spaced at, for example, 121/2 millisecond intervals.
A counter input/output circuit 30 is provided having an output
counter section for providing timed output pulses for energizing
the fuel injector 18 via a driver circuit 32 and an input counter
section responsive to speed pulses provided by a speed pickup 33
which senses rotation of the engine flywheel. In general, the output
counter section of the counter input/output circuit 30 may include
registers into which binary numbers representative of the desired
fuel pulse width are periodically inserted. Thereafter at times
determined by the microprocessor 20 the numbers are gated into
down counters which are clocked by clock pulses with the output
pulses of the output counter section having durations equal to the
time required for the down counters to be counted down to zero.
In this respect, the output pulse may be provided by a flip-flop
that is set when the number in the register is gated into the down
counter and reset by a carryout signal from the down counter when
the number is counted to zero. The input counter section of the
circuit 30 counts speed pulses from the pickup 33 for a predetermined
time period to measure the speed of the engine 10 or, alternatively,
counts clock pulses between input speed pulses to measure engine
speed.
To provide for the measurement of analog signals, the digital controller
includes a signal conditioner 34 which receives the various analog
signals and whose outputs are coupled to an analog-to-digital converter-multiplexer
36. The particular analog condition sampled and converted is controlled
by the microprocessor 20 in accord with the operating program stored
in the ROM 22 via address lines from the input/output interface
of an input/output circuit 38. Upon command, the addressed condition
is converted to digital form and supplied to the input/output circuit
38 and then stored in ROM designated memory locations in the RAM
24.
In the present invention, the inputs to the signal conditioner
34 which are utilized in determining the mass rate of air flow into
the engine 10 are a throttle position signal THP provided by a potentiometer
40 whose wiper arm is positioned by rotation of the throttle 16
and a manifold absolute pressure signal MAP provided by a pressure
sensor 42 sensing the manifold absolute pressure via a conduit 44.
Additional analog voltages may be applied to the signal conditioner
34 as required, for example in the control of the fuel injector
18. For example, a temperature signal representing engine coolant
temperature may be provided to the signal conditioner 34 to enable
control of fuel enrichment during cold engine operation.
The input/output circuits 30 and 38 are conventional circuits for
providing the respective functions. While the circuits have been
illustrated as being separate, they may be combined in one or more
input/output interface circuits.
The microprocessor 20 the ROM 22 the RAM 24 and the input/output
circuits 30 and 38 are interconnected by an address bus, a data
bus and a control bus. The microprocessor 20 accesses the various
circuits and memory locations in the ROM 22 and the RAM 24 via the
address bus. Information is transmitted between circuits via the
data bus and the control bus includes conventional lines such as
read/write lines, reset lines, clock lines, power supply lines,
etc.
Air flow through the orifice area defined by the throttle 16 and
into the engine 10 may be either sonic or subsonic depending upon
the engine operation. For sonic air flow through the orifice defined
by the throttle 16 the mass rate of air flow into the engine 10
is defined by the following expression: ##EQU1## where A is the
effective orifice area defined by the throttle 16 P.sub.1 is the
pressure above the throttle 16 and which is substantially equal
to atmospheric pressure, g is a constant which is the ratio of 1
slug to 1 pound, R is a gas constant, T is the absolute inlet air
temperature, and K is a specific heat ratio and is assumed to be
a constant for the normal range of ambient air temperature.
Assuming K to be a constant for a normal range of ambient temperature
air and assuming a constant air temperature, equation (1) can be
transformed to the expression: ##EQU2## where C.sub.S is the sonic
discharge coefficient that is equal to ##EQU3##
For subsonic air flow through the orifice defined by the throttle
16 the mass rate of air flow is defined by the expression: ##EQU4##
where C.sub.SS is the subsonic discharge coefficient, and P.sub.2
is the pressure downstream of the throttle 16 and which is substantially
equal to the absolute pressure in the manifold 12 of the engine
10. Equation 3 can be rearranged to obtain the following expression:
##EQU5## Assuming a constant temperature and by letting the constant
K.sub.1 equal the expression ##EQU6## in equations 2 and 4 the
equation for the mass rate of air flow into the engine 10 for sonic
air flow becomes:
and the equation for the mass rate of air flow into the engine
10 for subsonic air flow becomes: ##EQU7##
It can be seen that the expressions 5 and 6 for sonic and subsonic
flow differ from each other only in a pressure ratio dependent manner.
The discharge coefficient C.sub.S in the sonic expression holds
for sonic air flow which occurs when the pressure ratio attains
a critical value and the expression ##EQU8## varies only with the
pressure ratio P.sub.2 /P.sub.1. By defining a new function D which
is equal to C.sub.S for sonic air flow conditions and equal to the
expression ##EQU9## for subsonic air flow conditions, the mass rate
of air flow expressions for both sonic and subsonic air flows become
the single equation:
where D is an air flow driving function having a value dependent
upon the ratio P.sub.2 /P.sub.1. The value of D does not lend itself
to computation primarily because of the discharge coefficient terms
C.sub.S and C.sub.SS. However, the value of D can be determined
experimentally as a function of the pressure ratio P.sub.2 /P.sub.1.
FIG. 4 is illustrative of a schedule of values of D for values of
P.sub.2 /P.sub.1 varying from 0 to 1. By selecting values of D as
a function of the measured pressure ratio across the throttle 16
in accord with the schedule of FIG. 4 the mass air flow into the
engine 10 may be determined by the single equation 7 for both sonic
and subsonic air flow conditions. In the digital control system
of this embodiment, the schedule of values of D as a function of
the pressure ratio P.sub.2 /P.sub.1 is provided by a lookup table
in the ROM 22 that stores values of D at address locations which
are addressed by values of the pressure ratio P.sub.2 /P.sub. 1.
The operation of the digital system of FIG. 1 for determining the
mass rate of air flow into the engine 10 in accord with the principles
of this invention is illustrated in FIGS. 2 and 3. Referring to
FIG. 2 when power is first applied to the system such as upon closure
of the vehicle ignition switch (not illustrated), the computer program
is initiated at point 46 when power is first applied and then proceeds
to a step 48 where the computer provides for initialization of the
system. For example, at this step, system initial values stored
in the ROM 22 are entered into ROM designated locations in the RAM
24 and counters, flags, and timers are initialized. After the initialization
step 48 the program proceeds to step 50 where the program allows
interrupts to occur such as by resetting the interrupt mask bit
in the microprocessor condition code register. After the step 50
the program shifts to a background loop 52 which is continuously
repeated. This loop may include execution of routines such as diagnostic
and warning routines.
While the system may employ numerous program interrupts at various
spaced intervals, it will be assumed for purposes of illustrating
this invention that an interrupt A is provided at 121/2 millisecond
intervals by means of the divider 28 of FIG. 1 during which the
routine for determining the mass rate of air flow into the engine
10 is executed along with other routines such as the routine for
determining the fuel injection pulse width establishing a desired
air/fuel ratio.
Referring to FIG. 3 the mass air flow routine executed during
the interrupt A routine and which is repeated every 121/2 millisecond
intervals is illustrated. The routine is entered at point 54 and
proceeds to a step 56 where the computer executes a read routine
where the throttle position and manifold absolute pressure inputs
to the analog-to-digital converter/multiplexer 36 are each converted
into binary numbers representative of the analog signal values and
stored in respective ROM designated locations in the RAM 24. Also,
the engine speed value determined by the counter I/O 30 is read
and stored in the RAM 24.
The program next proceeds to a step 58 where a lookup routine is
executed to retrieve a stored pressure drop value from a lookup
table in the ROM 22 addressed as a function of the speed of the
engine 10 read and stored at step 56. The retrieved value is a predetermined
pressure drop across the throttle 16 when in a wide-open condition
at the existing engine speed. The pressure drop value is stored
in the RAM 24 and as subsequently described, will be utilized in
order to determine the barometric pressure which corresponds to
the pressure P.sub.1 in equation 7.
From step 58 the program proceeds to a decision point 60 where
it is determined whether or not the engine 10 is running. This is
accomplished by determining whether or not the engine speed read
at step 56 is greater than a predetermined value such as 0. If the
engine is not running, the manifold absolute pressure in the intake
manifold 12 is equal to the barometric pressure. When this condition
is detected, the program proceeds to a step 62 where the barometric
pressure stored in a ROM designated location in the RAM 24 is set
equal to the manifold absolute pressure read and stored at step
56. This value is thereafter utilized as the pressure P.sub.1 above
the throttle 16 in equation 7 in the determination computation of
the mass air flow into the engine 10. However, if the engine 10
is running so that the manifold absolute pressure is not a measure
of the barometric pressure, the program proceeds from decision point
60 to a decision point 64 where it is determined if the throttle
position signal represents a wide-open throttle condition. If this
condition exists, the program proceeds to a step 66 where the barometric
pressure value in the RAM 24 and which is utilized as P.sub.1 in
equation 7 is set equal to the sum of the manifold absolute pressure
value stored at step 56 and the pressure drop value stored in the
RAM 24 during the lookup routine of step 58.
Following step 62 and 66 or if the throttle position signal does
not indicate a wide-open throttle condition at decision point 64
the program proceeds to a step 68 where the manifold absolute pressure
value determined at step 56 is divided by the stored barometric
pressure value to provide the pressure ratio P.sub.2 /P.sub.1. From
step 68 the program proceeds to step 70 where the effective orifice
area A of the throttle 16 is obtained from a lookup table in the
ROM 22 as a function of the value of the throttle position signal
THP stored at step 56. Thereafter, the program proceeds to a step
72 where the air flow driving function lookup table in the ROM 22
containing the schedule of values of D is addressed in accord with
the pressure ratio P.sub.2 /P.sub.1 determined at step 68. The retrieved
value of the air flow driving function D, the effective orifice
area A of the throttle 16 determined at step 70 the pressure value
P.sub.1 determined at step 62 or step 66 and the stored value K.sub.1
are then used at step 74 where the mass rate of air flow is computed
to be equal to the expression AK.sub.1 P.sub.1 D. At point 76 the
program exits the routine.
In the foregoing manner, the mass rate of air flow into the engine
10 is determined without the addition of complex air flow sensing
elements in the air stream and without utilizing complex equations
requiring differentiation between sonic and subsonic air flows.
The foregoing description of a preferred embodiment for the purpose
of illustrating the invention is not to be considered as limiting
or restricting the invention since many modifications may be made
by the exercise of skill in the art without departing from the scope
of the invention. |